An action with stealthy (a ninja can never have a stealthy keyword themselves. only action can be stealthy) means that when that action is taken, if the ninja doing it is in stealth, they remain in stealth after the action. Most actions are not stealthy, like attacking, r running. Doing any of that automatically removes stealth. But a yajiri with stealthy on their ranged attack (or the str 1 kuni ranged attack every ninja has) will not remove a ninja from stealth, regardless of the attack outcome.